In the field of this invention techniques are known that use nanometer sharp tips for imaging and investigating the structure of materials down to the atomic scale. Such techniques include atomic force microscopy (AFM) and scanning tunneling microscopy (STM), as disclosed in EP 0 223 918 B1 and U.S. Pat. No. 4,343,993.
Based on the developments of the scanning tunneling microscopy and the atomic force microscopy, new storage concepts have been introduced over the past few years profiting from these technologies. Probes having a nanoscale tip are being introduced for modifying the topography and for scanning an appropriate storage medium. Data are written as sequences of bits represented by topographical marks, such as indentation marks and non-indentation marks. The tips comprise apexes with a radius in the lower nanometer range and the indentation marks have for example a diameter in the range of 20 to 40 nm. Hence, these data storage concepts promise ultra-high storage area density.
A storage device for storing data based on the AFM principle is disclosed in “The millipede—more than 1,000 tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, March 2000. The storage device has a read and write function based on a mechanical x-, y-scanning of a storage medium with an array of probes each having a tip. The probes operate in parallel, each probe scanning during operation an associated field of the storage medium. That way high data rates may be achieved. The storage medium comprises a thin polymethylmethacrylate (PMMA) layer. The tips are moved across the surface of the polymer layer in a contact mode. The contact mode is achieved by applying small forces to the probes so that the tips of the probes can touch the surface of the storage medium. For that purpose, the probes comprise cantilevers which carry the sharp tips on their end sections. Bits are represented by indentation marks or non-indentation marks in the polymer layer. The cantilevers respond to these topographic changes in the surface while they are moved across the surface. Indentation marks are formed on the polymer surface by thermomechanical recording. This is achieved by heating a respective probe with a current or voltage pulse during the contact mode in a way that the polymer layer is softened locally where the tip touches the polymer layer. The result is a small indentation on the layer having a nanoscale diameter.
Reading is also accomplished by a thermomechanical concept. The heater cantilever is supplied with an amount of electrical energy, which causes the probe to heat up to a temperature that is not high enough to soften the polymer layer as is necessary for writing. The thermal sensing is based on the fact that the thermal conductance between the probe and the storage medium, especially a substrate on the storage medium, changes when the probe is moving in an indentation as the heat transport is in this case more efficient. As a consequence of this, the temperature of the cantilever decreases and hence, also its electrical resistance changes. This change of electrical resistance is then measured and serves as the measuring signal.
In STM a sharp tip is scanned in close proximity to the surface and voltage applied between the tip and the surface gives rise to a tunnel current that depends on the tip-surface separation. From a data-storage point of view, such a technique may be used to image or sense topographic changes on a flat medium that represent a stored information in logical “0s” and “1s”. In order to achieve reasonable stable current, the tip-sample separation must be maintained extremely small and fairly constant. In STM, the surface to be scanned needs to be of an electrically conductive material.
Japanese Patent Abstract JP 08297870 discloses a data storage device, where a probe needle tip is brought into contact with a recording layer surface and a recording pulse voltage is applied. A current is made to flow between the probe and the substrate. The temperature of the recording layer is raised partially by the heat caused by current. This softens the recording layer. Succeedingly, the needle tip is pushed into the recording layer by a repulsive force to form the recording bit having a recessed structure.
WO 02/077988A2 discloses a method and an apparatus for writing data to and/or reading data from locations on a surface via a tip. The apparatus is designed for moving the tip between the locations on the surface. At each location, energy is selectively applied to the surface via the tip and the tip and the surface are selectively forced together in synchronization with the selective application of energy. Moving the tip into and out of contact with the surface is achieved by selective generation of a force field, which comprises an electric field.
WO 02/37488A1 discloses read/write components for AFM-based data storage devices. The read/write component comprises lever means and a support structure. The lever means is connected to the support structure for substantially pivotal movement. The lever means provides first and second current paths between a pair of electrical supply lines on the support structure, via which the lever means can be connected in use to power supply means operable in a write mode and a read mode. A write-mode heater is provided on the lever means in the first current path, and a read/write tip is provided on the write-mode heater. A read-mode heater is provided on the lever means in the second current path.
A fairly long life cycle for storage devices is important. Therefore, it is a challenge to provide a data storage device based on local probe techniques and a method for operating a storage device based on local probe techniques, which enables a reliable operation for a long period of time without significant medium or tip wear.